ISDN is a series of standards, defined by the International Telegraph and Telephone Consultative Committee (CCITT), which specify a network for offering digital communication services. These standards are found in the I, Q and X series of recommendations made by the CCITT.
A standard ISDN user-network interface is shown in FIG. 1 and comprises a local switch 2 (LS) and a network termination unit 4 (NT) which terminates a digital subscriber line 5 extended from the local switch 2. The network termination unit 4 is typically installed at a user's premises. A plurality of ISDN compatible devices 6 are coupled through corresponding plugs 8 to a local passive bus (PB) of metallic cable extending from the network termination unit 4. Two wires of the passive bus PB are used for signal transmission in the direction from the network termination unit 4 towards the ISDN devices 6 and the remaining two wires are used for signal transmission in the direction from the ISDN devices 6 towards the network termination unit 4.
The use of this type of ISDN technology provides an advancement in world-wide telecommunications by offering a new method for integrating voice and data information onto a single telephone line. ISDN's major feature is the ability to provide true point-to-point digital connectivity for transferring data and voice simultaneously. Using ISDN, several applications can be tied to the same line. These applications may include standard telephone, facsimile, computer, data conferencing, electronic and voice mail, to mention just a few.
Presently, there are several types of ISDN lines and protocols. A first type, known as a primary rate interface (PRI), offers, on a single ISDN line, twenty-three (23) data lines (called B lines or channels) and one administration line (called a D line or channel). The D Channel is used for signalling and data communications while the B Channels can be used for either digitally encoded voice or data communications. Another type of ISDN line is known as the basic rate interface (BRI) and is used to couple telephones, terminals, PCs and other desktop devices to higher order equipment and to the ISDN network. The BRI includes two B Channels and one D Channel (the B and D Channels are as described above). In both BRI and PRI, the B Channels operate at 64 kilo-bits per second (kbps), while the D Channel baud rates may vary depending upon the service, i.e., BRI or PRI.
Presently, however, ISDN is an emerging technology and, therefore, the necessary communication links for supporting ISDN compatibility are being targeted for large businesses. Generally, only these large enterprises have the ability to invest the time and money into developing ISDN implementations in their work place.
The targeting of large businesses for ISDN capability by the ISDN service providers generally omits the small to medium size businesses and consumers which are unable to obtain ISDN services in a cost efficient manner.